Method for signaling information by modifying modulation constellations

ABSTRACT

Methods and systems for communicating in a wireless network may distinguish different types of packet structures by modifying the phase of a modulation constellation, such as a binary phase shift keying (BPSK) constellation, in a signal field. Receiving devices may identify the type of packet structure associated with a transmission or whether the signal field is present by the phase of the modulation constellation used for mapping for the signal field. In one embodiment, the phase of the modulation constellation may be determined by examining the energy of the I and Q components after Fast Fourier Transform. Various specific embodiments and variations are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/929,142, filed Jun. 27, 2013, which is a continuation of U.S. patentapplication Ser. No. 12/319,191, filed Dec. 31, 2008, which is acontinuation of U.S. patent application Ser. No. 11/018,414, filed Dec.20, 2004, now issued as U.S. Pat. No. 7,474,608, which claims thebenefit of priority to U.S. Provisional Application No. 60/536,071,filed Jan. 12, 2004, all of which are hereby incorporated herein byreference in their entireties.

BACKGROUND OF THE INVENTION

In today's communications industry rapid advances in communicationprotocols and techniques are common. To facilitate widespread deploymentof new systems, significant efforts are often made to ensure newcommunications techniques and systems are compatible with previoussystems and devices, referred to herein as “legacy” systems or devices.

One problem associated with designing new generation systems is that, tobe compatible with legacy systems, new generation systems often have todeal with limitations inherent in the legacy systems. For example,preamble training and signaling fields of packets for legacy wirelesslocal area networks (WLANs) are already defined. To allow coexistencebetween legacy and new generation WLANs, it is desirable to preservepreambles having legacy compatible training and signaling fields.However, since legacy preambles may not be adequately designed todescribe new generation packet structures, which may have longer lengthsand/or require different training and signaling information, it can bechallenging to quickly identify which type of packet structure, e.g.,legacy or new generation, that follows a legacy compatible preamble.

Accordingly, a need exists to be able to quickly distinguish whether apacket having a legacy compatible preamble, may have a legacy packetstructure or a newer generation packet structure. Solutions to allowingcoexistence between legacy and new generation systems are thereforedesired without significantly complicating or constraining the signalingin new generation packet structures.

BRIEF DESCRIPTION OF THE DRAWING

Aspects, features and advantages of the embodiments of the presentinvention will become apparent from the following description of theinvention in reference to the appended drawing in which like numeralsdenote like elements and in which:

FIG. 1 shows block diagrams of two example packet structures for usewith wireless networks;

FIGS. 2a and 2b show respective graphs of different phases for amodulation constellation in order to distinguish packet structuresaccording to one embodiment of the present invention;

FIG. 3 is a flow diagram showing an exemplary method of communicatingaccording to one embodiment of the present invention;

FIG. 4 is a flow diagram showing a method of detecting types of packetstructure of a received transmission according to an embodiment of thepresent invention; and

FIG. 5 is a functional block diagram of an example embodiment for awireless apparatus adapted to perform one or more of the methods of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

While the following detailed description may describe exampleembodiments of the present invention in relation to wireless local areanetworks (WLANs), the invention is not limited thereto and can beapplied to other types of wireless networks or air interfaces whereadvantages could be obtained. Such wireless networks include, but arenot limited to, those associated with wireless wide area networks(WWANs) such as general packet radio service (GPRS), enhanced GPRS(EGPRS), wideband code division multiple access (WCDMA), code divisionmultiple access (CDMA) and CDMA 2000 systems or other similar systems,wireless metropolitan area networks (WMANs), such as wireless broadbandaccess systems including those supported by the WordwideInteroperability for Microwave Access (WiMAX) Forum, wireless personalarea networks (WPANs) and the like.

The following inventive embodiments may be used in a variety ofapplications including transmitters, receivers and/or transceivers of aradio system, although the present invention is not limited in thisrespect. Radio systems specifically included within the scope of thepresent invention include, but are not limited to, network interfacecards (NICs), network adaptors, mobile stations, base stations, accesspoints (APs), gateways, bridges, hubs and radiotelephones. Further, theradio systems within the scope of the inventive embodiments may includecellular radiotelephone systems, satellite systems, personalcommunication systems (PCS), two-way radio systems, two-way pagers,personal computers (PCs) and related peripherals, personal digitalassistants (PDAs), personal computing accessories and all existing andfuture arising systems which may be related in nature and to which theprinciples of the inventive embodiments could be suitably applied.

The following inventive embodiments are described in context of exampleWLANs using orthogonal frequency division multiplexing (OFDM) and/ororthogonal frequency division multiple access (OFDMA) although theinvention is not limited in this respect.

The Institute of Electrical and Electronics Engineers (IEEE) finalizeinitial standard for WLANs known as IEEE 802.11 (1997). This standardspecifies a 2.4 GHz operating frequency with data rates of 1 and 2 Mbpsusing either direct sequence or frequency hopping spread spectrum. TheIEEE 802.11 working group has since published three supplements to the802.11 standard: 802.11a (OFDM in 5.8 GHz band) (ISO/IEC 8802-11: 1999),802.11b (direct sequence in the 2.4 GHz band) (1999 and 1999Cor.-1/2001), and 802.11g (OFDM in the 2.4 GHz band) (2003). Thesesystems most notably 802.11a and 802.11g utilizing OFDM, areindividually or collectively referred to herein as “legacy” WLANs.

The IEEE 802.11a standard specifies an OFDM physical layer that splitsan information signal across 52 separate sub-carriers to providetransmission of data. The primary purpose of the OFDM Physical Layer isto transmit MAC (medium access control) protocol data units (MPDUs) asdirected by the 802.11 MAC Layer. The OFDM Physical Layer is dividedinto two elements: the PLCP (physical layer convergence protocol) andthe PMD (physical medium dependent) sublayers. The PLCP sublayerprepares MAC protocol data units (MPDUs) for transmission and deliversincoming frames from the wireless medium to the MAC Layer. The PLCPsublayer minimizes the dependence of the MAC layer on the PMD sublayerby mapping MPDUs into a frame format (also referred to as packetstructure) suitable for transmission by the PMD.

Examples of frame formats or packet structures 100 for use in WLANs aregraphically represented in FIG. 1 and may include a preamble portion fora receiver to acquire an incoming OFDM signal and synchronize thedemodulator. The preamble may include one or more training fields and/orsignaling fields (sometimes separately referred to as headers)including, for example, a legacy short training field (L-STF), a legacylong training field (L-LTF) and a legacy signaling field (L-SIG) 114,124, which are collectively referred to herein as a legacy compatiblepreamble. The portion of packet structures 100 to follow the legacycompatible preamble may depend on whether the packet structure is alegacy packet structure 110 or a newer generation packet structure 120.

For legacy packet structures 110 one or more data fields 112 typicallyfollow the legacy compatible preamble and the rate and length (in OFDMsymbols) of the legacy packet structure 110 may be determined by areceiver from the values present in the L-SIG field 114 of the legacypreamble. However, the L-SIG field 124 may not be sufficient to describenew generation packet structures 120, such as those currentlycontemplated for adoption in the IEEE 802.11n standard for highthroughput (HT) WLAN. By way of example, reverse bits in the L-SIG fieldmay already be used by legacy devices for other purposes. Accordingly,additional signaling and/or training, generally depicted by HT-SIG block122, may be needed to define the HT packet structure and/or synchronizethe demodulator to handle the HT modulation.

However, since an L-S field 114, 124 may be present in all legacycompatible preambles; it may be difficult for a receiver to know whetherlegacy data 112 follows the signaling field 114 or whether additional HTsignaling or training 122 follows the signaling field 124.

The long training symbols (L-LTF) that immediately precede the signalfield 114, 124 allow a receiver to accurately estimate the clock phaseso that demodulation of the signal field, for example, using binaryphase shift keying (BPSK), is possible.

In generating OFDM signals, encoded and/or interleaved bits may bemapped on a transmit modulation constellation, for example,constellations for BPSK, quaternary phase shift keying (QPSK), and/orvarious quadrature amplitude modulation (QAM) modulation schemes. Aninverse Fast Fourier Transform (FFT) may then be performed on the mappedcomplex values to generate an array of complex values to produce an OFDMsymbol and for which multiple symbols are joined together to produce anOFDM frame. On the receiving end, an FFT is performed to retrieve theoriginally mapped complex values which are then demapped using thecorresponding constellation and converted back to bits, decoded, etc.

Turning back to FIGS. 2a and 2b , in accordance with one embodiment whenthe packet has a legacy packet structure (e.g., an IEEE 802.11astructure 110; FIG. 1), a traditional modulation constellation such asBPSK constellation 210 of FIG. 2a may be used for mapping complex valuesfor one or more fields (e.g., 114, 112) of the legacy packet structure.Further, when the packet has a newer generation structure (e.g., IEEE802.11n structure 120; FIG. 1) the one or more fields (e.g., 124, 122)may be modulated using a modified modulation constellation such as aBPSK constellation 220 having a phase rotation of 90 degrees as shown inFIG. 2b . Of course, the modified constellation 220 could be used forsignaling legacy packet structures and traditional constellation 210could be used for signaling new generation packet structures if desired.In this manner, information may be signaled to a receiver withoutmodifying preamble structures or fields of the packets themselves.

Constellation 220 may be referred to as a BPSK-Q or Q-BPSK constellationsince its coordinates (+1, −1) are positioned along the Q axis asopposed to traditional BPSK constellation 210 having coordinates (+1,−1) along the I axis.

The 90 degree rotation of a BPSK constellation is effective as it has nosignificant effect on the robustness of the packet field (e.g., signalfield) with the modified modulation technique. However, the phaserotation for mapping values of a modulation constellation does not haveto be 90 degrees and/or other types of modulation constellations such asthose used for QPSK modulation and the like could also be used.Consequently, the inventive embodiments are thus not limited to anyparticular modulation constellation or degree of phase rotation.

Turning now to FIG. 3, a method 300 for transmitting in a wirelessnetwork may include modulating 325 one or more positions of atransmission using a modulation constellation having a modified phase inorder to signal a receiving device of a type of packet structureassociated with the transmission.

In certain embodiments, method 300 may include encoding bits 305 andinterleaving 310 the encoded bits. If 315 a legacy packet structure isto be transmitted, one or more of the packet fields may be modulated 320using traditional modulation constellations, such as a BPSKconstellation (210; FIG. 2). On the other hand, if 315 a new generationpacket structure is to be transmitted, one or more of the packet fieldsmay be modulated 325 using a modified modulation constellation, such asa Q-BPSK constellation (220; FIG. 2).

In certain example embodiments, there may be two types of packetstructures, a legacy packet structure substantiality in conformance withan IEEE 802.11a type packet structure and a second packet structuresubstantially in conformance with an IEEE 802.11n type packet structure.In one example implement only the HT-SIG field (122; FIG. 1) of an HTpacket structure may be modulated using Q-BPSK however, the embodimentsare not limited in this manner. Further, in certain implementations,signaling a packet type using phase rotated modulation constellationsmay only be used for packets which have a data payload.

For a receiver, the decision about whether the signal field is a legacymodulation or a HT field could be made by examining the amount of energyin the I and Q components after the FFT. For example, if the Q energy isgreater than the I energy (the threshold for which may be set assuitably desired), then the receiver may determine the packet has anHT-SIG field. Otherwise it may be a legacy packet or vice versa. Sincethis decision can utilize all data modulated subcarriers, for example,at least 48 for a 20 MHz WLAN system, this affords a 17 dB processinggain resulting in a highly reliable decision. The proposed detectionscheme may only be applied to the data modulated subcarriers and pilotsubcarriers can be handled differently if desired.

Turning to FIG. 4, a method 400 of receiving a wireless network mayinclude determining a type of packet structure associated with anincoming transmission based on an I and Q energy levels of a respectivebaseband signal.

In certain embodiments, method 400 may include performing 405 a FFT on areceived transmission and examining 410 I and Q components after theFFT. If 415 the Q energy is significantly greater than the I energy, theassociated packet field is determined 420 to be an HT-SIG field.Otherwise it is identified 425 as being a legacy packet. The FFT valuesmay then be demapped using the corresponding modulation constellationsand converted back to bits, decoded, etc.

In an example implementation, the I and Q energy levels are used todetermine whether a phase of a binary phase shift keying (BPSK)constellation used to map the HT-SIG field has been rotated although theembodiments are not limited in this respect.

Turning to FIG. 5, an example apparatus 500 for use in a wirelessnetwork may include a host processing circuit 550 may be any componentor combination of components and/or machine readable code adapted toperform one or more of the methods described herein. In one exampleimplementation, circuit 550 may include a baseband processing circuit553 to modulate bits for at least a portion of a transmission using amodulation constellation having a modified phase in order to signal areceiving device of a type of packet structure associated with atransmission. Alternatively or in addition, baseband processing circuit553 may configured to detect energy levels of data modulated subcarriersas previously described. Apparatus 500 may also include a medium accesscontroller circuit 554 and/or a radio frequency (RF) interface 510 ifdesired.

Host processing circuit 550 and/or RF interface 510 may include anyhardware, software and/or firmware components necessary for physical(PHY) link layer processing and/or RF processing of respectivereceive/transmit signals for supporting the various air interfaces.

Apparatus 500 may be a wireless mobile station such as a cell phone,personal digital assistant, computer, personal entertainment device,wireless router, a network access station such as a WLAN access point(AP) or other equipment and/or wireless network adaptor therefore.Accordingly, the functions and/or specific configurations of apparatus500 could be varied as suitably desired.

The components and features of apparatus 500 may be implemented usingany combination of discrete circuitry, application specific integratedcircuits (ASICs), logic gates and/or single chip architectures. Further,the features of apparatus 500 may be implemented using microcontrollers,programmable logic arrays and/or microprocessors or any combination ofthe foregoing where suitably appropriate.

It should be appreciated that apparatus 500 shown in the block diagramof FIG. 5 is only one functionally descriptive example of many potentialimplementations. Accordingly, division, omission or inclusion of blockfunctions depicted in the accompanying figures does not infer that thehardware components, circuits, software and/or elements for implementingthese functions would necessarily be combined, divided, omitted, orincluded in embodiments of the present invention.

Embodiments of apparatus 500 may be implemented using single inputsingle output (SISO) systems. However, certain alternativeimplementations may use multiple input multiple output (MIMO)architectures having multiple antennas 518, 519.

Unless contrary to physical possibility, the inventors envision themethods described herein may be performed in any sequence and/or in anycombination; and the components of respective embodiments may becombined in any manner.

Although there have been described example embodiments of this novelinvention, many variations and modifications are possible withoutdeparting from the scope of the invention. Accordingly the inventiveembodiments are not limited by the specific disclosure above, but rathershould be limited only by the scope of the appended claims and theirlegal equivalents.

The invention claimed is:
 1. A method for transmitting packetscomprising encoded bits, the method comprising: selecting one of eitherconvolutional encoding or low-density parity-check (LDPC) encoding basedon a high-throughput signaling (HT-SIG) field of a packet; configuring aforward-error correction (FEC) coding field of the HT-SIG field of thepacket to indicate whether the packet is encoded with the LDPC encodingor the convolutional encoding; encoding payload data for the packet inaccordance with the selected encoding; configuring the packet to be ofvariable-length in high-throughput (HT) format to include at least theHT-SIG field and a data field, the data field to include the encodedpayload data; and transmitting the packet in accordance with anorthogonal frequency division multiplexing (OFDM) format.
 2. The methodof claim 1 wherein transmitting comprises transmitting the packet inaccordance with one of a plurality of orthogonal frequency divisionmultiplexing (OFDM) formats, and wherein the OFDM formats include a 20MHz OFDM format, a 40 MHz HT OFDM format, or a 40 MHz duplicate OFDMformat.
 3. The method of claim 2 further comprising selecting the OFDMformat from one of the OFDM formats of the plurality.
 4. The method ofclaim 2 wherein encoding the payload data comprises selecting an encoderfor the encoding based on the FEC coding field, wherein a single LDPCencoder is selected for LDPC encoding, wherein either one or twoconvolutional encoders are selected for convolutional encoding, andwherein when two convolutional encoders are selected for convolutionalencoding, the method included dividing data bits between the twoconvolutional encoders.
 5. The method of claim 2 wherein for encodingvariable length packets, the method includes indicating a payload lengthin a payload length field in a high-throughput signaling field.
 6. Themethod of claim 5 wherein when LDPC encoding is selected, the methodincludes: encoding the payload data with a shortened LDPC codeword forpayloads at or below a first predetermined length; encoding the payloaddata with two LDPC codewords that have been shortened equally forpayloads above the first predetermined length and at or below a secondpredetermined length; and for payloads above the second predeterminedlength, encoding a number of bytes of the payload data one or moreunshortened LDPC codewords and encoding remaining bytes of the payloaddata using one or more shortened LDPC codewords.
 7. The method of claim1 further comprising: performing channel interleaving when convolutionalencoding is selected; and refraining from performing channelinterleaving for selected modulation types when LDPC encoding isselected.
 8. The method of claim 1 wherein selecting comprises selectingLDPC encoding for higher performance situations and selectingconvolutional encoding for lower performance situations.
 9. The methodof claim 8 wherein the higher performance situations include one or moreof increased data rates, higher order modulations, adaptive bit loading(ABL), OFDM MIMO operation, longer distance transmissions, and longerpacket lengths, and wherein lower performance situations include one ormore of decreased data rates, lower order modulations, non-ABL, non-OFDMMIMO operation, shorter distance transmissions, and shorter packetlengths.
 10. The method of claim 1, wherein the method is performed byan access point operating in an IEEE 802.11 configured network fortransmitting encoded data to one or more communication stations (STAs),wherein the FEC coding field is a one-bit field, and wherein thetransmitted packet is a packet protocol data unit (PPDU).
 11. An accesspoint comprising: processing circuitry to select one of eitherconvolutional or low-density parity-check (LDPC) encoding based on ahigh-throughput signaling (HT-SIG) field of a packet; and physical layer(PHY) circuitry to: configure a forward-error correction (FEC) codingfield of the HT-SIG field of the packet to indicate whether the packetis encoded with the LDPC encoding or the convolutional encoding; encodepayload data for the packet in accordance with the selected encoding;configure the packet to be of variable-length in high-throughput (HT)format to include at least the HT-SIG field and a data field, the datafield to include the encoded payload data; and transmit the packet inaccordance with an orthogonal frequency division multiplexing (OFDM)format.
 12. The access point of claim 11 the packet is transmitted inaccordance with one of a plurality of orthogonal frequency divisionmultiplexing (OFDM) formats, and wherein the OFDM formats include a 20MHz OFDM format, a 40 MHz HT OFDM format, or a 40 MHz duplicate OFDMformat.
 13. The access point of claim 12 wherein the processingcircuitry is further configured to select the OFDM format from one ofthe OFDM formats of the plurality.
 14. The access point of claim 12wherein the processing circuitry is further configured to select anencoder for the encoding based on the FEC coding field, wherein a singleLDPC encoder is selected for LDPC encoding, wherein either one or twoconvolutional encoders are selected for convolutional encoding, andwherein when two convolutional encoders are selected for convolutionalencoding, the method included dividing data bits between the twoconvolutional encoders.
 15. The access point of claim 12 wherein forencoding variable length packets, a payload length is indicated in apayload length field in a high-throughput signaling field.
 16. Theaccess point of claim 15 wherein when LDPC encoding is selected:encoding the payload data is encoded with a shortened LDPC codeword forpayloads at or below a first predetermined length; the payload data isencoded with two LDPC codewords that have been shortened equally forpayloads above the first predetermined length and at or below a secondpredetermined length; and for payloads above the second predeterminedlength, a number of bytes of the payload data are encoded with one ormore unshortened LDPC codewords and remaining bytes of the payload dataare encoded using one or more shortened LDPC codewords.
 17. The accesspoint of claim 11 wherein the PHY circuitry is configured to: performchannel interleaving when convolutional encoding is selected; andrefrain from performing channel interleaving for selected modulationtypes when LDPC encoding is selected.
 18. The access point of claim 11wherein the processing circuitry is further configured to select LDPCencoding for higher performance situations and convolutional encodingfor lower performance situations.
 19. The access point of claim 18wherein the higher performance situations include one or more ofincreased data rates, higher order modulations, adaptive bit loading(ABL), OFDM MIMO operation, longer distance transmissions, and longerpacket lengths, and wherein lower performance situations include one ormore of decreased data rates, lower order modulations, non-ABL, non-OFDMMIMO operation, shorter distance transmissions, and shorter packetlengths.
 20. A non-transitory computer-readable storage medium thatstores instructions for execution by one or more processors to performoperations for transmitting encoded packets, the operations comprising:selecting one of either convolutional encoding or low-densityparity-check (LDPC) encoding based on a high-throughput signaling(HT-SIG) field of a packet; configuring a forward-error correction (FEC)coding field of the HT-SIG field of the packet to indicate whether thepacket is encoded with the LDPC encoding or the convolutional encoding;encoding payload data for the packet in accordance with the selectedencoding; configuring the packet to be of variable-length inhigh-throughput (HT) format to include at least the HT-SIG field and adata field, the data field to include the encoded payload data; andtransmitting the packet in accordance with an orthogonal frequencydivision multiplexing (OFDM) format.